The present invention relates to an analogue frontend according to the preamble of claim 1.
Analogue frontends such as are used, for example, in telecommunications receivers comprise various analogue components which are arranged upstream of an analogue/digital converter. The analogue components, which may be, for example, amplifiers or filters, are supplied with an analogue reception signal which is processed by the analogue components before it is fed to the analogue/digital converter for sampling and digitization.
Precise drive level control of the analogue frontend is already very important and will become even more important in the future for the following reasons.
As the structural widths of integrated circuits become smaller and smaller the clock frequencies which are used on these integrated circuits continue to rise. This has the consequence that the analogue components are increasingly subjected to interference from the digital components located on the respective integrated circuit and the integrated circuits arranged in the direct vicinity. It can therefore be expected that the interference effects caused by noise will become more and more pronounced. In contrast, the supply voltage to the analogue components will decrease as the structural widths become smaller so that the distance between the increasing noise element and the decreasing supply voltage will be reduced. The decreasing structural widths cause the digital components to become smaller and smaller. Because the analogue components cannot be reduced in size in the same way, their significance is increasing. For this reason, it is important to control the drive level of the analogue components in an optimum way in order to keep the implementation losses as low as possible.
Hitherto, so-called peak detectors have been used to control the drive level of the analogue components which are not located directly upstream of the analogue/digital converter. Whenever the analogue signal exceeds a specific threshold, this transgression is perceived by the peak detectors. If the number of peaks detected by the peak detectors within a specified time interval exceeds a specific limiting value, the drive level of the analogue components is reduced. If, on the other hand, an excessively low number of peaks is detected by the peak detectors, the drive level of the analogue components is increased.
However, controlling the drive level of the analogue frontend and/or of the corresponding analogue components using peak detectors entails a number of problems.
Firstly, the region between the largely linear behaviour of the analogue frontend and the signal distortion caused by the amplitude limiting is fluid. That is to say there are regions in which the analogue signal is not distorted at all upstream of the analogue components, distorted little, greatly distorted or very severely distorted. However, with the previously described peak detectors just one of these thresholds can be interrogated or monitored. However, this threshold would have to be changed as a function of the amplitude histogram of the analogue input signal in order to be able to control the drive level of the analogue frontend in an optimum way. However, when digital television signals are received with satellite support, the amplitude histogram of a pure QPSK (xe2x80x9cQuadrature Phase Shift Keyingxe2x80x9d) signal, for example, differs to a relatively large extent from that of an SCPC (xe2x80x9cSingle Carrier Per Channelxe2x80x9d) signal.
As has been described above, the peak detectors register the transgression of a predefined signal amplitude in a region in which the analogue signal of the analogue frontend is distorted to a greater or lesser degree. Of course, this transgression may take place only very rarely so that the overall distortion of the analogue signal is sufficiently small. The peak detectors should, with respect to the clock of the analogue/digital converter, be triggered once to approximately the order of magnitude of each 10,000-th sampled value, so that in each case a drive level control of the analogue components is carried out after every 10,000 sampled values. However, this means that the measuring periods become very long and the variance of the measurement is relatively high.
The invention is therefore based on the object of proposing an analogue frontend in which a more precise drive level control of the analogue components, in particular of the analogue components which are not located directly upstream of the analogue/digital converter can be carried out with only a small degree of additional expenditure.
This object is achieved according to the invention by means of an analogue frontend having the features of claim 1. The subclaims describe respective preferred and advantageous embodiments of the present invention.
In the solution according to the invention the fact is exploited that the analogue frontend which is usually embodied in the form of an integrated circuit, already has with the analogue/digital converter a very precise measuring instrument for determining amplitudes. The analogue/digital converter can optionally be connected downstream of each stage, i.e. downstream of each analogue component, of the analogue frontend in order to measure the analogue signal level at the different switched positions of the analogue signal level. The output signal of the analogue/digital converter is subsequently evaluated in order to generate a drive level control signal for the respective analogue component.
Every sampled value of the analogue/digital converter can be used for this purpose in the phase of level readjustment of the analogue frontend because in this phase the content of the received analogue signal is not of interest so that the analogue frontend, and thus the analogue received signal, can be set quickly and precisely. On the other hand, in the phase of ongoing operation, the content of the received signal is of primary interest. Nevertheless, the signal levels of the analogue frontend must be monitored because the reception conditions can change at any timexe2x80x94for example as a result of adjacent channels in the respective receiver being switched on or off. For this reason, during ongoing operation, preferably only every k-th sampled value of the analogue/digital converter is used to check the drive level control of the analogue frontend at different switching points. The sampled value which is not available for the digital output signal owing to this measure can be determined by interpolation from adjacent sampled values of the analogue/digital converter.
The analogue frontend according to the invention can thus be used to control the drive level of each individual analogue component reliably and precisely. It is possible to determine a histogram etc. of the analogue input signal at each point on the analogue frontend in order to be able to ensure a corresponding drive level control of the analogue components which can change the level or the amplitude response of the analogue input signal. It is not necessary to use peak detectors. If control is not carried out with respect to the peak values but rather, for example, with respect to the variance of the input signal, the control or drive level control can react more quickly because now every k-th sampled value, instead of every 10,000-th sampled value, contains the amplitude information which is necessary for drive level control.
The invention is suitable in principle for precise drive level control of any analogue frontend of any design. However, the present invention is suitable in particular for analogue frontends such are used in receivers for telecommunications applications, for example cable modems.